This invention relates to fiber optic transmission systems and, more particularly to bidirectional systems operating at a single wavelength and to photodiodes for use in such systems.
Bidirectional transmission of optical signals over a fiber optic link between terminals at remote locations may be accomplished using a pair of fibers, one for transmission in each direction. Although this system configuration allows simultaneous transmission in both directions (full-duplex) and provides a high degree of isolation between the two transmission paths (or channels), it is disadvantageous because it duplicates transmitters and receivers for each fiber and consequently increases cost and consumes space.
The problem of duplicating fibers is alleviated in a wavelength-multiplexed, bidirectional system which utilizes only a single fiber and specially designed terminals. Unfortunately, the transceivers at either end of such a fiber optic link are not identical; that is, at one end the light source emits outgoing radiation at .lambda..sub.1 and the photodetector detects incoming radiation at .lambda..sub.2 .noteq..lambda..sub.1 ; whereas at the other end a different photodetector detects the radiation at .lambda..sub.1, and a different light source emits radiation at .lambda..sub.2. Another problem is the need for optical couplers at each end to connect both the light source and photodetector to the single fiber. Nevertheless, transmission at different wavelengths in each direction does provide good isolation between the two transmission channels.
An alternative prior art single-wavelength scheme sacrifices some of the isolation of the wavelength-multiplexed system for simplicity of terminal design. In this type of arrangement, transmission in both directions over a single fiber takes place at the same wavelength in each channel. Thus, the transceivers at each terminal can be identical. However, radiation transmitted in one direction can be reflected in the opposite direction by a number of light-reflecting interfaces in the system; e.g., at fiber connectors or at the remote photodetector. In a full-duplex system the reflected radiation is a source of noise or crosstalk for radiation transmitted in the same direction. In addition, these systems as currently proposed utilize some type of optical beam splitter to couple radiation from the fiber to the photodetector and from the light source to the fiber. Each pass of the radiation through the beam splitter entails at least a 3 dB loss, thus the total system loss (transmission plus reception) is at least 6 dB and in practice is 7-8 dB.